Parasitic diseases in fishes seriously limit aquaculture production and its economic viability in waters of tropical and subtropical areas, both in wild and culture systems (Kensley 2001). Several facultative and obligatory parasites belonging to order Isopoda, family Aegidae, Class Maxillopoda are capable of tremendous damage leading to growth retardation, emaciation, anemia and often death of the infested fish (Carrasson and Cribb 2014). Although parasite infestation may not result in immediate death, the repulsive appearance of the fish infested with parasites will cause significant economic losses by reducing its market value (Lima et al. 2013; Eiras and Castro, 2016). A. typus is a notorious and active ectoparasite which infects fresh and brackish water fish fauna (Ravichandran et al. 2009). In the present study, A. typus was found attached to the skin, gills and opercular chambers of C. striata, C. catla, C. carpio and W. attu caught from capture and culture facilities of the fresh and brackish waters along the southwest coast of India. Although it is reported to infest fishes such as Anabas testudineus, Puntius sarana, Channa orientalis, Lebistes reticulates, Tilapia mossambica, and Macropodus cupanus (Nair et al. 1983), in the present study, infestation was not found in any of these fish species and other carps caught along with the four species of fish mentioned above.
According to Baillie et al. (2019), parasite attachment structure and area of attachment are critical traits in host exploitation and survival. It was observed in the present study that the parasite was having long and slender body tapering towards the ends and the efficient contour of the body offers minimal resistance to the water flow. The dactylus of the pereopod were modified with sharp edges to facilitate attachment. It is reported that the isopod, when found in the gill cavity, can impair respiration by causing gill atrophy due to the pressure it exerts on the gills, and in the mouth cavity, it can impede normal feeding by causing changes in the oral structures of the host like wasting of tongue (Kabata, 1985; Lester, 2005; Ravichandran et al. 2007; Kerry et al. 2017). Gill attached parasites feeds on blood from epithelial tissues, causing a reduction in the total erythrocyte count and hemoglobin content that result in anemia (Nair and Nair 1983). Hematological studies were not done in the present study. However, the infested fishes appeared to be very weak and emaciated with pale gills, thus showing symptoms of anaemia. It was also found that the favourite site of the parasite for attachment and feeding was the gills, as most of the parasites were seen within the gill chamber of the infested fishes regardless of the species of the host. Frequent shifts in position as the parasite feeds on the host results in serious wounds that make the host more vulnerable to secondary disease-causing agents (Lima et al. 2013; Eiras and Castro 2016; Watchariya and Songsuk 2020). In the present study too, there were wounds on the body of the infested fishes, which may be because of this behaviour of the parasites.
The histopathological studies of the gills showed rupture of the lamellae, epithelial uplifting, vacuolization and infiltration of blood cells. The pressure exerted by the parasite on the gills and attachment of parasites with their hooked pereopods may be the reasons for the observed pathological changes including rupture of the lamellae. In addition, haemocytic infiltration is primarily caused by increased blood flow as a host defense mechanism. The present observation correlates with the results of histopathological studies done by Ravichandran et al. (2007) and Mohammadi et al. (2012) on the gills of Joryma tardoor, Astronotus ocellatus and Symphysodon spp infested with isopods. Lifting of the respiratory epithelium is one of the earliest injuries found in fish and it is characterized by the displacement of the lining epithelium of the secondary lamellae which leads to edema (Santos et al. 2014).
The average mean intensity and prevalence have a significant impact on host survival. According to Aydodgu et al (2015) water temperature, water pollution, parasite biology, the hormonal status, immune response, migration and feeding habits of the host and the availability of intermediate host might influence the parasite intensity and prevalence. The mean intensity of infestation in the present study was highest in W. attu and lowest in C. carpio. One of the important factors determining the mean intensity and prevalence of parasites infesting each fish is the surface area of the gills and opercular chamber (Ravichandran et al. 2009). The surface area of the gills in W. attu is greater than in other fishes, which is most likely the cause of the high mean intensity. More than three parasites were frequently found on the gills of W. attu, whereas only one to two parasites were found on the gills of C. striata. A low mean intensity of infestation in fish population appears to pose no danger, but the whole population can get infested within no time, especially in semi intensive and intensive culture facilities. In the present study, C. striata had the highest prevalence of infestation and W. attu had the lowest (Fig. 5). Although low prevalence of infestation does not appear to impact the economic viability, it alters fish behavior, anatomy and morphology (Sasal 2005; Ravi and Rajkumar 2007) and also, makes the fishes more prone or susceptible to secondary infections.
Water temperature is a key variable of the aquatic ecosystem influencing many aspects in aquatic organisms and pathogen existence in terms of growth, host susceptibility, disease transmission, increase in pathogen development and survival rate (Mallick and Panigrahi et al. 2018). There exists a relation between the environmental parameters and outbreak of parasitic diseases, although it is not always direct (Short et al. 2017). Many studies have found temperature to be one of the most important variables controlling the elevational distribution of parasites (Oommen and Shanker 2005; Meik and Lawing 2008; Marcohliese 2016). It is also reported that the seasonal distribution of helminth parasites is influenced by temperature (Hanzelova and Zintan, 1985). In the current study a direct correlation was found between temperature and the prevalence of A. typus (Fig. 7). There are several reports that supports the finding where rising temperatures accelerate the replication rate and virulence of parasites affecting the economic and social viability of aquaculture (Marcohliese 2008; Collins et al. 2020). The decrease in the abundance of the parasites with the onset of monsoon in the present study, may be attributed to reduced water temperature due to heavy rainfall.
There are no specific or effective and environmentally safe chemotherapeutic measures available to control this parasite other than manual removal of parasites and management through optimal implementation of management practices based on good water quality, nutrition, sanitation, and quarantine. For pond treatment, a chemical treatment with trichlorfon at a concentration of 0.5 – 0.75 ppm for 24 h is recommended by Chinabut (2002). Emamectin benzoate, which is very effective against crustacean parasites, is also being used to treat isopods in freshwater areas of Andhra Pradesh at a dose rate of 300-400 mg/kg feed for 3-5 days, despite the fact that no scientific data is available regarding this treatmentve. However, unregulated use of such chemicals can have adverse effect on fish and environment (Wang et al. 2011). Rejeena et al (2021) has suggested that the usage of bacterial consortium with high chitinase, protease, lipase and urease activity, can be an effective biological control for A. typus infestion in fishes. Another method of biological control is to use plant extracts and introduction of predators like crabs (Shah et al. 2017).
In the face of global warming and fluctuating environmental changes, incidences of parasite infestations on fish are expected to become more frequent. Further studies are needed for finding effective control measures for parasites and for responsible chemical use for the safety of animals and environment, the sustainability of commercial aquaculture production and also for the human wellbeing.